Electrophotography (or xerography) is now in rapidly widespread use in a printing apparatus (e.g., a duplicator, a facsimile machine, and a printer). The electrophotography comprises digitizing an image to be copied, applying a laser beam to a photoconductor (or a photoreceptor) in response to the digital signal to form an electrostatic latent image on the photoconductor, and then developing the latent image using charged toner. For the electrophotographic apparatus, various roll brushes or bar brushes are used. As the fiber constituting these brushes, fibers having characteristics according to the purposes are used. Into the electrophotographic apparatus, for example, the following brushes are incorporated: a brush for applying a solid lubricant (such as zinc stearate) to easily remove a toner from a photoconductor; a cleaning brush for physically or electrostatically removing a residual toner after printing in a photoconductor, a charge roller, a transfer roller, or a transfer belt; a charging brush for applying a voltage on a toner to charge the toner either of positive or negative; a static removal brush for removing an electrostatic charge from a charged body; and others. Among these brushes, an electro-conductive brush (e.g., a cleaning brush, a charging brush, and a static removal brush) requires a specified electric conductivity. In particular, the cleaning brush usually requires a stable resistance value of the order of 109 Ω/cm.
As an electro-conductive fiber for the electro-conductive brush, various electro-conductive fibers have been reported. For example, for the cleaning brush, an electro-conductive fiber in which an electro-conductive agent is added to a regenerated fiber (such as a viscose rayon fiber) or a synthetic fiber (e.g., a polyamide fiber, a polyester fiber, an acrylic fiber, and a polypropylene fiber) is used.
Moreover, a duplicator or a printer is now tending to be downsized and speeded up. In addition, there is a tendency to make a particle size of a toner smaller. In order to suit these tendencies, improvement in various brushes is also required. In particular, the cleaning brush is required to show more uniform electric resistance value and to contain a thinner filament as a single filament of a constitute fiber thereof. At the same time, the cleaning brush also requires a high durability (e.g., less settling and less variable electric resistance value).
As an example of a production of a fiber by mixing an electro-conductive carbon into a synthetic resin, for example, Japanese Patent Application Laid-Open Publication No. 2007-247095 (JP-2007-247095A, Patent Document 1) discloses a conductive polyester fiber which comprises a multifilament yarn composed of a plurality of single filaments of a polyester resin, wherein the single filaments contain 15 to 25% by mass of a carbon black having an average particle size of 15 to 40 nm and a DBP oil absorption of 130 to 200 cm3/100 g, and the multifilament has an electric resistance value of 1×104 to 9×109 Ω/cm.
Japanese Patent Application Laid-Open Publication No. 2003-306832 (JP-2003-306832A, Patent Document 2) discloses a polyamide multifilament comprising an electro-conductive carbon, the polyamide multifilament containing magnesium in an amount of 8 to 25 mmol in 1 kg of the polyamide multifilament and having a specific resistance value of 103 to 108 Ω/cm.
When each one of these electro-conductive fibers is used to produce an electro-conductive yarn having the order of 109 Ω/cm as required for the cleaning brush, however, there is a large variation in the resistance value among the yarns or in the resistance in the length direction of the yarn. Thus a brush comprising the yarn has an uneven resistance value, which causes unevenness of electrostatic removal of a toner. Further, since each one of these electro-conductive fibers contains an electro-conductive carbon in a polymer of the fiber, the fiber has a low flowability in melt spinning and lack spinnability. There is therefore a variation in the fineness of the single filaments composed of the obtained multifilament, which induces unevenness in the physical removal of the toner. Further, the single filament, which has a large fineness, easily scratches a photoconductor.
In order to make a fiber from a polymer containing an electro-conductive carbon mixed thereinto and having a low spinnability, a method which comprises forming the polymer and a polymer having a high spinnability into a conjugated fiber has also been reported. For example, Japanese Patent Application Laid-Open Publication No. 2006-9177 (JP-2006-9177A, Patent Document 3) discloses an electro-conductive conjugated fiber having a cross section which comprises a component A comprising a polymer containing an electro-conductive carbon and a component B comprising a polymer incompatible with the component A, wherein the component A is divided to a plurality of segments by the component B. This document discloses a conjugated fiber in which a core comprising the component B and having a 3- to 10-pointed star-shaped cross section is covered with the component A. The fineness of the component B segment is 3 to 7 dtex.
Japanese Patent Application Laid-Open Publication No. 2008-196073 (JP-2008-196073A, Patent Document 4) discloses a conductive conjugated fiber; the conductive conjugated fiber comprises a non-conductive component composed of a polyester resin and a conductive component composed of a polylactic acid containing 10 to 25% by mass of a carbon black having an average particle size of 15 to 35 nm and a DBP absorption of 40 to 150 cm3/100 g, exhibits that at least part of the conductive component is exposed to the surface of the fiber, and has an electric resistance value of 1×109 Ω/cm to 9×1012 Ω/cm. In Examples of this document, a conductive conjugated fiber having a fineness of 28 dtex/2 filaments is obtained.
For each one of these electro-conductive conjugated fibers, however, even if the fineness of the single filament is smaller, the fineness limit of the conjugated fiber is 3 dtex. It is substantially difficult to produce a conjugated fiber having a fineness smaller than 3 dtex. Further, the problem of the large variation in the resistance value among the yarns or in the resistance value in the length direction of the yarn in the electro-conductive yarn having the order of 109 Ω/cm required for the cleaning brush also arises for the conjugated fiber.